Radiation therapy is a common form of cancer treatment that involves the use of certain types of energy (called ionizing radiation) to kill cancer cells and shrink tumors. There are two main types of radiation therapy, internal radiation (brachytherapy) and external radiation (systemic).
Systemic therapy includes radiation that is delivered from a device external to the patient, and is typically delivered from a linear accelerator, a synchrotron, or a cyclotron, and can be used to treat most types of solid tumors. Typically, external radiation therapy is performed over multiple sessions to maximize the amount of radiation delivered to the tumor and minimize the amount delivered to the surrounding healthy tissue.
The radiation delivered to a patient is called radiation dose or dosage. A radiation dose is measured using a unit called a gray (Gy) and/or centigrays (cGy). Different tissues and organs are capable of withstanding different amounts of dosage and still functioning correctly. For example, the liver can withstand approximately 3,000 cGy, while the kidneys can only withstand approximately 1,800 cGy.
Planning and delivery of radiation therapy typically involves many different team members. Four of these members may include a radiation oncologist, a dosimetrist, a radiation physicist, and a radiation therapist. A radiation oncologist is a doctor who specializes in the treatment of cancer using radiation and typically participates in many steps of the treatment process. The dosimetrist determines the proper radiation dose for the patient. The radiation physicist, who may participate in both the planning and treatment phases, ensures that the equipment delivers the prescribed amount of radiation to the proper location. The radiation therapist, who also typically participates in the planning and treatment phases, delivers the actual treatment.
During a typical planning stage, a patient lies still on a table while the therapist uses an x-ray machine to define the treatment locations (based on patient contours and tumor location(s). A CT (Computed Tomography) device or an MRI (Magnetic Resonance Imaging) device is typically used in this stage to identify the location of the tumor and identify surrounding healthy tissue and organs. During the treatment process, the areas where radiation will be delivered are marked, such as with a temporary or permanent marker, tiny dots, and/or a tattoo. These locations are then defined for future sessions. For some forms of treatment, especially where the patient may be likely to move, molds may be made to ensure that the patient remains motionless.
Once a treatment plan has been devised, it is typically tested to ensure that the radiation delivered by the plan matches the prescribed dosage from the radiation oncology, which is at least one goal of radiation dose calculation. If there is too little dosage delivered to the target site a tumor may persist, and if there is too high a dosage delivered healthy tissue may be harmed.